1. Field of the Invention
The present invention relates to a knocking control apparatus and method and an engine control unit for a variable cylinder internal combustion engine which can be switched between a full cylinder operation mode for operating all of a plurality of cylinders and a partial cylinder operation mode for pausing some of the plurality of cylinders by a cylinder pausing mechanism.
2. Description of the Prior Art
Conventionally, Laid-open Japanese Patent Application No. 7-103060, for example, describes a knocking control apparatus for a variable cylinder internal combustion engine. In this internal combustion engine, all of four cylinders are operated in a full cylinder operation mode, while two of the four cylinders are paused by a cylinder pausing mechanism in a partial cylinder operation mode. The internal combustion engine is also provided with a knocking sensor for detecting knocking. The knocking sensor detects vibrations of the internal combustion engine for each cylinder to output a knocking detection signal. This knocking control apparatus determines based on the output knocking detection signal whether the knocking signal indicates knocking or seating noise which can be associated with opening/closing of intake and exhaust valves. Specifically, when the locking detection signal has a vibration level equal to or higher than a threshold, it is determined that the knocking is occurring. On the other hand, when the vibration level is lower than the threshold, the knocking detection signal is determined to represent seating noise. Then, when it is determined that the knocking is occurring, a knocking control procedure is performed to correct a basic ignition timing, which has been set based on an operating condition of the internal combustion engine, toward the retarding side in order to suppress the knocking.
This knocking control apparatus is also configured to set the threshold for the partial cylinder operation mode to be larger than the threshold for the full cylinder operation mode, thereby avoiding erroneous determination which could be made if a single threshold is set in both the operation mode.
As described above, the conventional knocking control apparatus for a variable cylinder internal combustion engine determines knocking or seating noise in accordance with a vibration level detected by the knocking sensor, whether or not it is equal to or higher than the threshold. However, the foregoing knocking control apparatus has a problem of the inability to appropriately set the threshold if it cannot control the engine in a set operation mode, for example, due to a fault in a cylinder pausing mechanism, and cannot determine the number of actually operated cylinders.
For example, if the engine cannot normally return to the full cylinder operation mode due to a fault in the cylinder pausing mechanism which disables the engine to switch from the partial cylinder operation mode to the full cylinder operation mode, the number of actually operated cylinder will be less than the essential number of operated cylinders in the full cylinder operation mode. Therefore, if the knocking control apparatus uses a lower threshold for the full cylinder operation for the determination of knocking in this state, the knocking control apparatus can erroneously determine seating noise as knocking even though no knocking is actually occurring. In this event, the erroneous detection of knocking would erroneously correct the ignition timing toward the retarding side, possibly resulting in higher temperatures of exhaust gases and earlier deterioration of catalyst.
On the other hand, when the cylinder pausing mechanism fails in a manner reverse to the above, where the knocking control apparatus cannot switch the engine from the full cylinder operation mode to the partial cylinder operation mode, all the cylinders are continuously operated without correctly pausing appropriate cylinders. When the knocking control apparatus uses a larger threshold for the partial cylinder operation mode for the determination of knocking in this state, the knocking control apparatus fails to determine knocking, even though it is actually occurring, and therefore can fail to correctly detect the knocking, contrary to the foregoing. In this event, the ignition timing is not corrected toward the retarding side, while the knocking is occurring, and is continuously controlled toward the advancing side, eventually resulting in engine stall.